Blockade of DNA synthesis induced by platelet-derived growth factor by tranilast, an inhibitor of calcium entry, in vascular smooth muscle cells

The present study was conducted to establish a pharmacological method of controlling growth of vascular smooth muscle cells (VSMC) by blocking calcium entry. In cultured rat VSMC, 1 nM platelet-derived growth factor (PDGF) induced a biphasic elevation of cytoplasmic free calcium concentration, ([Ca²⁺](c)). The second sustained phase of [Ca²⁺](c) was dependent on extracellular calcium. At lower concentrations, PDGF induced oscillatory changes in [Ca²⁺](c), and reduction of extracellular calcium attenuated the oscillation. An antiallergic compound, tranilast, abolished the sustained phase of [Ca²⁺](c) induced by 1 nM PDGF. Tranilast also inhibited the oscillatory changes in [Ca²⁺](c) induced by 200 pM PDGF. In addition, PDGF- induced calcium influx in the late G₁ phase, as assessed by measuring the initial uptake of ⁴⁵Ca, was inhibited by tranilast in a concentration- dependent manner. Tranilast also inhibited PDGF-augmented DNA synthesis; the ID₅₀ for the inhibition of DNA synthesis was nearly identical to that for calcium influx. Although tranilast blocked PDGF-induced calcium entry, it did not affect PDGF-mediated autophosphorylation of the PDGF receptor, activation of phosphatidylinositol 3-kinase, activation of Ras or mitogen-activated protein kinase. Similarly, PDGF-induced elevation of diacylglycerol was not affected by tranilast. These results suggest that the antiallergic drug tranilast inhibits PDGF-induced DNA synthesis by blocking PDGF-mediated calcium entry. Tranilast may be of use in controlling PDGF-induced DNA synthesis in VSMC.